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Digest - IEEE Antennas and Propagation Society. International Symposium

Multi-antenna channel estimation for key establishment in rician propagation

Britton T. Quist, Michael A. Jensen

Published Jul 1, 2013• 0 citations
Publisher

Abstract

This paper focuses on a recently-developed technique for maximizing the number of encryption key bits that can be established using multi-antenna estimation of the reciprocal multipath electromagnetic propagation channel. The analysis shows that for Rician channels with a strong line-of-sight component, the performance of the original technique is degraded. A modified approach is then proposed to allow effective key establishment over a broad range of propagation conditions.

DOI
10.1109/aps.2013.6710938

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•Jan 1, 2018•0 citations
This paper focuses on a recently-developed technique for maximizing the number of encryption key bits that can be established using multi-antenna estimation of the reciprocal multipath electromagnetic propagation channel. The analysis shows that for Rician channels with a strong line-of-sight component, the performance of the original technique is degraded. A modified approach is then proposed to allow effective key establishment over a broad range of propagation conditions.
The performance of techniques for establishing secret encryption keys from the reciprocal electromagnetic propagation between two wireless radios is generally measured by comparing the number of key bits generated to the key rate, or the information theoretic bound on the number of key bits that can be determined for a single independent channel estimate. While typically this bound is computed by assuming that the radios have no information regarding the channel, recent work has demonstrated an approach for computing this bound for nodes using beamformed antenna arrays that know the spatial covariance of a wide-sense stationary channel. Since this bound does not directly indicate how to perform practical channel estimation to obtain estimates that can achieve the bound, the prior work demonstrates an approximation that indicates a practical channel estimation procedure while yielding a mutual information near that of the key rate in rich multipath fading environments. This paper demonstrates appreciable degradation in the mutual information achieved using this approximation when the channel includes a line-of-sight component and proposes a modified approximation technique that performs well over a broad range of propagation conditions.
While secret encryption keys are commonly-used for securing wireless transmissions, establishment of such keys has traditionally been accomplished using computational techniques at the higher layers of the wireless communication protocol stack. However, the multipath electromagnetic propagation typical of many environments can also be an excellent source of random variables useful for encryption key generation. Furthermore, antennas play an important role in how the transmitter and receiver excite or sense the channel and therefore become a critical component in the performance of techniques that exploit the multipath channel for key establishment. This paper focuses on how using multiple antennas can dramatically improve the performance of such key establishment methods.
Generation of secret encryption keys from the reciprocal electromagnetic channel represents an emerging topic within the broad field of propagation channel research. This paper uses multi-antenna channel measurements to assess the potential performance of such techniques, with the emphasis on using a reconfigurable antenna element controlled by the channel sounder to create time-variation in the wireless channel that enables establishment of long encryption keys. The results demonstrate that reconfigurable antennas can significantly enhance the security, even when the eavesdropper antennas are adjacent to or surround one of the legitimate nodes.
Propagation-based key establishment is a physical layer method for generating encryption keys based on two radios observing a reciprocal propagation channel. This work explores the impact of mutual coupling when communicating nodes are equipped with multiple antennas, multipath richness in the propagation environment, and practical limitations caused by the nonreciprocal nature of RF circuits on key establishment. First, network theory is used to formulate a model of a realistic communication system which incorporates transmit sources and receive loads, impedance matching networks, low-noise amplifiers (LNAs), mutually coupled antenna arrays, and a passive eavesdropper. Afterwards, a detailed analysis is performed to quantify the impact of coupling, type of impedance matching network, and proximity of a multi-antenna eavesdropper on key rate metrics. Next, the degradation on key establishment caused by the radiocircuitry non-reciprocal contributions to the propagation channel is analyzed. A calibration technique based on total least square algorithm is used to overcome the non-reciprocity. Results demonstrate that the method is highly effective in removing the impact of non-reciprocal circuit contributions over a range of operational parameters. Lastly, for key establishment, the propagation conditions can cause the available key rate to be significantly different from the secure key rate which takes into account the presence of a passive eavesdropper. To study this in detail, a realistic multiple-input multiple-output (MIMO) propagation environment is modeled for two communicating radios and an eavesdropper. Afterwards different propagation conditions are assumed and results demonstrate that secure key rate converges to available key rate when K-factor is small and the eavesdropper is not located very close (< 2.5 wavelength) to one of the nodes.